In Japan, only 30% of the primary energy supply is used as effective energy, with about 70% being eventually lost to the atmosphere as heat. The heat generated by combustion in industrial plants, garbage-incineration facilities or the like is lost to the atmosphere without conversion into other energy. In this way, we are wastefully discarding a vast amount of thermal energy, while acquiring only a small amount of energy by combustion of fossil fuels or other means.
To increase the proportion of energy to be utilized, the thermal energy currently lost to the atmosphere should be effectively used. For this purpose, thermoelectric conversion, which directly converts thermal energy to electrical energy, is an effective means. Thermoelectric conversion, which utilizes the Seebeck effect, is an energy conversion method for generating electricity by creating a difference in temperature between both ends of a thermoelectric material to produce a difference in electric potential. In this thermoelectric generation, electricity is generated simply by setting one end of a thermoelectric material at a location heated to a high temperature by waste heat, and the other end in the atmosphere (room temperature) and connecting conductive wires to both ends. This method entirely eliminates the need for moving parts such as the motors or turbines generally required for power generation. As a consequence, the method is economical and can be carried out without releasing gases due to combustion. Moreover, the method can continuously generate electricity until the thermoelectric material has deteriorated.
Therefore, thermoelectric generation is expected to play a role in the resolution of future energy problems. To realize thermoelectric generation, large amounts of a thermoelectric material that has a high thermoelectric conversion efficiency and excellent heat resistance, chemical durability, etc. will be required.
CoO2-based layered oxides such as Ca3CO4O9 have been reported as substances that achieve excellent thermoelectric performance in the air at high temperatures. However, all such oxides have p-type thermoelectric properties, and are materials with a positive Seebeck coefficient, i.e., materials in which the portion located at the high-temperature side has a low electric potential.
To produce a thermoelectric module using thermoelectric conversion, usually not only a p-type thermoelectric material but also an n-type thermoelectric material are needed. However, n-type thermoelectric materials that have excellent heat resistance, chemical durability, etc., and have a high thermoelectric conversion efficiency have not yet been found. Therefore, thermoelectric generation using waste heat has not yet become practical.
In such circumstances, the development of n-type thermoelectric materials that are composed of abundantly available elements and have excellent heat resistance, chemical durability, etc., and have a high thermoelectric conversion efficiency is greatly desired.